Three-dimensional articles produced by rapid prototyping techniques (e.g. stereolithography) tend to increase in brittleness over time. The problem is particularly acute for articles made from resin systems that cure (at least in part) by a cationic mechanism. In general, the cationic reaction initiated by actinic radiation (e.g. a UV laser) continues indefinitely after the radiation ceases. This is known as the “dark reaction” of cationically curable systems. Paul F. Jacobs, Stereolithography and Other RP&M Technologies 33-35 (ASME Press 1996); Christian Decker & Khalil Moussa, Kinetic study of the cationic photopolymerisation of epoxy monomers, 28 J. Polym. Sci, Polym. Chem. 3429-43 (1990). The cationic reactions continue at an ever slower rate as the amount of unreacted material decreases. As the reaction continues over time, the article becomes more and more brittle, elongation at break decreases, and tensile strength and tensile modulus increase.
Today, there is a growing need for rapid prototyping materials and processes that produce three-dimensional articles with stable tensile properties. In the past, articles produced by rapid prototyping were initially so brittle that increasing brittleness associated with aging went unnoticed. Also, the issue of brittleness was often never confronted because articles were used only in short-term applications.
Once the problem was realized, persons of skill in the art tried to solve it by formulating cationically curable resin systems so as to produce articles with very high initial values for elongation at break and impact strength, in the hopes that useful tensile properties would persist over time. However, this approach brought about compromises in other performance metrics, with the result that overall resin performance was often sacrificed. Other attempts to solve the problem were based on the observation that epoxy resins to pick up ambient moisture. Water contained in the polymer matrix acts as a plasticizer and tends to mask brittleness. However, this approach is hard to control because it depends on ambient humidity and dimensional accuracy suffers.
There is, therefore, a need in the art of rapid prototyping for a photocurable composition that produces three-dimensional articles with stable tensile properties. Today's need, if fulfilled, would result in the cost-effective use of rapid prototyping techniques to produce three-dimensional articles suitable for multiple applications over time. The present invention relates to photocurable compositions comprising high molecular weight, low functionality components that can be used to produce three-dimensional articles with stable tensile properties and thus improved end uses.